1. Field of the Invention
The invention relates to a fuel injection system for internal combustion engines and to a method for injecting fuel into the combustion chamber of an internal combustion engine.
2. Description of the Prior Art
As exhaust gas standards become more and more stringent, ever higher injection pressures are demanded to improve mixture formation and combustion. The result is greater mechanical and thermal stresses on the fuel injection system. In addition, the demand for drive capacity increases disproportionately, since with the pressure, the losses in the fuel injection system rise as well.
In a fuel injection system known from German Published, Nonexamined Patent Application DE-OS 197 38 804, a pressure step-up means is connected between the injection pump and the injection nozzle. As a result, the full injection pressure is applied only in the region around the injection nozzle. Fuel supply is effected through a bypass, directly from the highpressure region of the injection pump. When the pressure is increased in the injection pump, but also upon flow through the bypass, the fuel heats up greatly, which has negative impacts on the compressibility of the fuel and on its density.
The object of the invention is to furnish a fuel injection system in which the thermal stress on the injection pump is reduced and the possible pressure increase rates in the fuel injection system are improved. In addition, higher injection pressures should be made possible, and at the same time the stress on the injection pump and demand for drive power of the injection pump should be reduced.
According to the invention, the above objects are attained by a fuel injection system for internal combustion engines, having an injection nozzle and having an injection pump that has a high-pressure part, the high-pressure part of the injection pump being operatively connected to the injection nozzle via a control line, communicating with a low-pressure side of a pressure step-up means, and via a highpressure path communicating with a high-pressure side of the pressure step-up means, and a feed line is present which feeds fuel to the injection nozzle. A first check valve disposed in the feed line prevents the reverse flow of fuel from the injection nozzle into the feed line, and the feed line communicates with a low-pressure supplier.
This fuel injection system has the advantage that the injection pressure is applied only between the high-pressure side of the pressure step-up means and the injection nozzle. At the same time, the pressure forces acting on the injection pump are reduced. As a result, the leakage and throttling losses are reduced as well, which leads to a reduction in the demand for drive capacity and improves the hydraulic efficiency of the fuel injection system. Furthermore, the fuel in the high-pressure region of the fuel injection system remains comparatively cold, since it is delivered directly from the low-pressure part of the injection pump. As a result, the compressibility of the fuel is less, which results in an improved pressure increase rate in the fuel injection system, and a greater mass flow can be pumped through the injection nozzle. In addition, the thermal and hydraulic improvement of the fuel injection system makes smaller injection port diameters of the injection nozzle possible, which improves the mixture formation at all operating points.
In one embodiment of the invention, the pressure step-up means has a step-up piston, which is displaceable in a bore and whose end faces each define one pressure chamber, whose first, larger end face of the step-up piston defines a first pressure chamber communicating with the control line, and whose second, opposed, smaller end face of the step-up piston defines a second pressure chamber, communicating with the high-pressure path.
In a further embodiment of the invention, it is provided that the feed line communicates with the second pressure chamber, so that the fuel is introduced in the part of the high-pressure region farthest away from the injection nozzle and is pumped from there as far as the injection nozzle. This has the advantage that in the highpressure region of the fuel injection system, the fuel is continuously replaced with relatively cold fuel.
Another embodiment provides that a first check valve is disposed in the feed line and prevents the reverse flow of fuel from the injection nozzle into the feed line, so that the low-pressure supplier of the injection pump is not acted upon by the injection pressure.
In one embodiment of the invention, the first check valve is spring-loaded, so that with maximum reliability under all operating conditions, the reverse flow of fuel from the injection nozzle into the feed line is prevented.
A further embodiment provides that the change in cross section of the step-up piston and a shoulder in a housing of the pressure step-up means define a relief chamber, so that possible leakage losses of the pressure step-up means can be collected and carried away.
In another embodiment of the invention, it is provided that the relief chamber communicates through a connecting line with the part of the feed line that is located between the low-pressure supplier and the first check valve, so that the leakage from the pressure step-up means is returned to the fuel injection system.
In one embodiment of the invention, a restoring spring is fastened in the relief chamber, is braced on a stationary support and in the process acts on the step-up piston at the change in cross section toward the relief chamber and as a function of the standing pressure in the control line, the end faces of the step-up piston, and the opening pressure of the first check valve presses the step-up piston against its stop toward the pump between injections, so that when the control line is pressure-relieved, the step-up piston is brought quickly, and independently of the pressure in the feed line, to its outset position. Moreover, the restoring spring requires only little installation space.
In a further embodiment of the invention, in the connecting line between the relief chamber and the feed line, a second check valve is provided, which blocks the communication in the direction from the feed line to the relief chamber, so that the feed line is not excited by the pressure fluctuations in the relief chamber.
A further embodiment of the invention provides that a scavenging valve designed as a check valve with a blocking direction from the control line to the feed line is disposed between the control line and the feed line, so that as soon as the pressure in the control line drops below the pressure in the feed line, filling of the control line is achieved through the scavenging valve. This leads to a drop in the temperature level in this region as well and thus improves the hydraulic performance of the fuel injection system and lessens the danger of seizing in the injection pump.
Further in the invention, it is provided that the scavenging valve does not open until an adjustable pressure difference between the control line and the feed line is reached, so that the motion of the step-up piston to its outset position is supported in this version as well by the pressure in the feed line, and the filling of the control line in the region between the injection pump and the pressure step-up means, which is difficult above all at high rpm, is assured since at high rpm the pressure in the feed pump is high as well.
A further embodiment of the invention provides that the part with the larger end face of the step-up piston upon which the pressure of the control line acts when the step-up piston is resting on its stop toward the pump is larger than the smaller end face of the step-up piston; that in the feed line between the first check valve and the injection pump, a third check valve with the same blocking direction is provided; and that between the control line and the first and third check valves, a connecting line with a fourth check valve with a blocking direction from the feed line to the control line is provided, so that at the beginning of injection, bypassing the pressure step-up means, fuel is pumped from the high-pressure part of the injection pump directly into the injection nozzle. As a result, the pressure increase rate changes at the onset of injection, and as a result the combustion noise can be abated, and furthermore it becomes easier to meter small preinjection quantities by means of provisions taken at the pump.
In a further feature of the invention, it is provided that the third and fourth check valves are combined into a bypass valve, so that the number of components is reduced, thus reducing expenses.
In another variant of the invention, the low-pressure supplier is part of the injection pump, so that the number of component groups is reduced, and a drive is required only for the high-pressure part of the injection pump and for the low-pressure supplier.
Further in the invention, a step-up piston embodied in two parts is provided, thus improving production, assembly and the hydraulic properties of the injection system.
In a further embodiment of the invention, it is provided that at least two injection nozzles are present; that one control line and one pressure step-up means each are disposed between each injection nozzle and the injection pump; and that all the injection nozzles communicate with the low- pressure supplier via feed lines.
The object stated at the outset is also attained by a method for injecting fuel into the combustion chamber of an internal combustion engine, by means of the fuel injection system in which
a pressure relief of the control line takes place between injections;
fuel is pumped out of the low-pressure supplier via the feed line to the injection nozzle;
the step-up piston is moved to its stop toward the pump; and
the fuel injection is controlled by the high-pressure part of the injection pump.
In this method, the full injection pressure is applied only directly in front of the injection nozzle; the maximum injection pressure is increased, and at the same time the load on the injection pump from pressure forces and temperatures is reduced. Furthermore, because of the reduced leakage and reduced throttling losses, the hydraulic efficiency of the system is improved and thus the requisite drive capacity is reduced further. The low temperature makes steeper pressure increases possible, because of the reduced elasticity of the fuel, and for the same pumping quantity, a higher flow rate through the nozzle is made possible. The thermal and hydraulic improvements in the fuel injection system allow smaller injection port diameters of the injection nozzles and thus better mixture formation at all operating points.
In a modification of the method of the invention, it is provided that until an adjustable pressure difference between the control line and the high-pressure side of the pressure step-up means is reached, the fuel injection is controlled, bypassing the pressure step-up means, by the high-pressure part of the injection pump; and that above the adjustable pressure difference between the control line and the high-pressure of the pressure step-up means, the fuel injection is controlled by the high-pressure part of the injection pump with the aid of the pressure step-up means. This method has the advantage that because of the different injection rate at the onset of injection, combustion noise is abated, and the metering of small preinjection quantities becomes easier through provisions made in the pump.